It is known in prior art to use ribbons in thermal transfer imaging processes. In these processes, thermal means are used to selectively heat areas of ribbon having an image transfer layer or coating. The printing is generally achieved by heat transferring the coating from the ribbon to paper by this local heating of the ribbon. Such image-localized heating may be accomplished by contacting the ribbon with point electrodes and a broad area contact electrode. The high current densities in the area of the electrodes during the applied voltage produces intense local heating which causes transfer of the coating from the ribbon to paper or receiving medium adjacent to or in contact with the ribbon. Various publications such as "IBM Technical Disclosure Bulletin, Resistive Ribbon Thermal Transfer Printing Method", Crooks, et al., vol. 19, No. 11, Apr. 1977, p. 4396 illustrate this general thermal transfer technique. Printers and some other hardware used in these methods are disclosed in U.S. Pat. Nos. 4,326,812; 4,327,365 and 4,329,071.
Various ribbons have been suggested for use in these thermal processes. These ribbons usually contain a resistive layer, a conductive layer and a hot melt ink layer. When the current is selectively applied to the ribbon in image configuration, the resistive material heats up causing the ink at that point to transfer to the printing surface. It has been found that after flexing and continued use of this ribbon, cracking and loss of the ink layer frequently occurs. This cracking is caused primarily because of relatively poor adhesion of the ink layer to the Mylar or other substrate used.
There have been suggested several varieties and combinations of ink layers and substrates in an attempt to improve adhesion and durability. U.S. Pat. Nos. 2,713,822 and 3,744,611 both describe non-impact printing processes employing a ribbon ink layer and ribbon substrate. The ink layer generally comprises a mixture of carbon black or dye and waxes. Many of the early ink layers had relatively poor rub resistance and layer adhesion. Several improved ribbons have been proposed which provide beneficial properties over the earlier used ribbons. Some of these improved ribbons are disclosed in U.S. Pat. Nos. 4,172,064; 4,269,892; 4,291,994; 4,308,318; 4,309,117 and 4,320,170. Several ink layers contain water-based coatings such as described in U.S. Pat. No. 4,172,064. Although water-based coatings have substantial advantages, the practical difficulties of forming good coatings from water-borne systems are not easily overcome. Since most organic polymer systems that would be expected to be candidates to form attractive coatings are not soluble in water, the organic phase must be present as a latex, i.e. as a colloidal macromolecular stabilized suspension. The polymer itself must demonstrate superior film characteristics after the solvent has evaporated. To do this the film formation process must proceed through an evaporative process depositing the organic polymer as well as forming a coherent film by the coalescence of the discrete polymer particles. Similarly, the polymer itself must be capable of film coalescence during the evaporation and, once formed, must be tough and resistant to both detergents and solvents. Finally, the polymer system chosen should be capable of being made industrially, since not every polymer precursor can be combined with others in proportions that will give the physical properties that are desired in the resulting polymer system.
In order to form a coherent film on a substrate from a latex, a coalescing agent is usually incorporated. These materials are usually ether-alcohol compounds with typical materials being methyl, ethyl or butyl CELLOSOLVE, butyl CARBINOL, butyl carbinol acetate and the like. The function of the coalescing agent is to soften and reduce the viscosity of the non-aqueous phase so that the individual particles fuse together to form the required continuous film as the aqueous phase disappears during drying. The coalescing agent may be slowly volatile and will thus leave the film after it has formed. Since the interaction between the coalescing agent and the polymer phase must occur during the drying process and at the polymer-water and polymer-polymer interfaces, the optimum coalescent agent tends to be a specific component for a given system. The specific coalescent agent is chosen to be one which functions best with the specific copolymer system being used to form the coherent film.
Because the aqueous coating compositions are used to coat substrates that may or may not be easily wet by water, a surfactant system should be incorporated. The surfactant must be compatible with any surfactant system that may already be present at the polymer surface, being present both as emulsifier and stabilizer from the polymerization process. Similarly, the surfactants must not interfere with the coalescing agent in its fusion role. The surfactant lowers the surface tension of the aqueous system so that the coating formulation wets both the printing plate or device, as well as the substrate onto which the coating is being applied. Under these conditions the printing or coating can be applied by conventional equipment already in use in the trade. Since these printing devices are often rotating cylinders which dip into the aqueous material while rotating at high speed, the surfactants chosen must be effective but must not produce excessive foam either at the supply fountain or at the surface as the printing or coating transfer of material occurs.
In a practical coating system, there are also employed several auxiliary ingredients so-called because they have only minor effects on the physical properties intrinsic to the polymeric coating. These added materials are vital to a useful product. They include pigments to give the coating color and opacity, anti-foam agents to reduce foaming, anti-freeze components to give the system freeze-thaw resistance, and fungicides and mildewcides to minimize degradation. There may also be present ultraviolet light stabilizers and anti-oxidants and, while all of these components would be most important and necessary for a useful product and must be chosen so that they would not interact either with the basic polymer system or with each other, they have minor effect on the intrinsic properties of the coating system itself.
In U.S. Pat. No. 4,269,892 a ribbon having a transfer coating and a novel substrate containing a polyester resin containing from about 15% to 40% by weight of electrically conductive carbon black is disclosed. In U.S. Pat. No. 4,291,994 a tear resistant ribbon for thermal transfer processes is disclosed. The substrate is made from a polycarbonate, a block copolymer of bis-phenol A carbonate and dimethyl siloxane. A novel substrate made from polyurethane is disclosed in U.S. Pat. No. 4,320,170. These three patents 4,269,892; 4,291,994 and 4,320,170 disclose new substrates for use in thermal transfer processes while patents 4,172,064; 4,308,318 and 4,309,117 disclose novel ink layers use in thermal transfer processes. The balance attempted to be achieved in each of these above-discussed patents is compatibility of substrate and ink layer to achieve maximum long-range use. While all of these proposed ribbons present a variety of improvements, they generally do not have sufficient or at least good enough adhesion to the Mylar or other substrates to permit crinkling of the thermal transfer ribbon without some cracking and loss of effectiveness of the ink layer.